This invention relates to a process for producing a methacrylic acid ester.
One problem in the production of methacrylic acid esters is that a time required for filtration for removing a catalyst and the like is long.
For example, Japanese Patent Unexamined Publication No. 54-61117 proposes a process for producing a methacrylic acid ester by a transesterification reaction using lithium hydroxide as a catalyst, but the time for requiring filtration for removing the catalyst and the like is long. This seems to be caused by the fact that most of the catalyst lithium hydroxide is converted to a lithium salt of methacrylic acid, which is a very fine powder and causes clogging of the filter to prolong the filtration time.
Another problem in the production of methacrylic acid esters is that large bubbles are generated in a reactor during the reaction to cause a flooding phenomenon in a rectifying column by the bubbles, resulting in making the operation difficult.
As is well known in the art, a methacrylic acid ester is produced by a process wherein methyl methacrylate and an alcohol are subjected to a transesterification reaction in the presence of a transesterification catalyst and a polymerization inhibitor. The transesterification reaction of methyl methacrylate and an alcohol is an equilibrium reaction and can be usually proceeded by removing produced methanol by azeotropic distillation with methyl methacrylate. Further, in order to enhance the reaction rate and suppress the production of polymers, it is general to use methyl methacrylate in an excess amount with regard to the alcohol, and to remove the methyl methacrylate by distillation after the transesterification reaction, followed by a purifying step such as filtration, distillation, and the like. For this reason, a batch type operation is usually employed for the production of methacrylic acid esters. Further, the transesterification reaction is generally carried out by using a tank type reactor equipped with a stirrer and a rectifying column for azeotropic distillation of methyl methacrylate and methanol.
In order to carry out the transesterification reaction with industrially and economically high productivity, it is necessary to maintain an evaporating amount of methanol from the reactor more than the prescribed amount in order to reduce the methanol content in the reactor. Further, in order to prevent the loss of methyl methacrylate by distillation, it is necessary to properly control a reflux ratio at the rectifying column so as to make the mixture of methyl methacrylate and methanol to be distilled from the rectifying column near the azeotropic composition (methanol content 92% by mole) depending on the methanol concentration in the reactor.
The present inventors carried out the transesterification reaction for the production of methacrylic acid esters under proper evaporation amounts and reflux ratio as mentioned above, but large bubbles generated in the reactor to cause a flooding phenomenon in the rectifying column by the bubbles, resulting in making the operation difficult. In order to conduct the reaction smoothly, it was necessary to add methyl methacrylate and the alcohol in a volume of 45 to 60% of the capacity of the reactor, which resulted in reducing the yield and remarkably damaging the productivity.
A still further problem in the production of methacrylic acid esters is that when a secondary or tertially alcohol is used as a starting material, catalytic activity is lowered and separation of the catalyst by filtration is not easy.
Various catalysts have been studied in processes for producing methacrylic acid esters by a transesterification reaction of methyl methacrylate and an alcohol.
For example, it is proposed to use an acidic catalyst such as sulfuric acid or p-toluenesulfonic acid (Japanese Patent Examined Publication Nos. 48-21929 and 48-37011). But since the acidic catalyst is remarkably small in the activity, a long period of time is necessary for the reaction, and there easily takes place side reactions such as intramolecular dehydration reaction, and the like.
On the other hand, as a basic catalyst, an alkali metal alkoxide such as sodium methoxide is generally used (British Patent No. 976,304). This catalyst has defects in that the catalytic activity is lowered during the reaction and there easily takes place a side reaction of addition of the starting alcohol or by-produced alcohols to double bonds of the starting ester and the desired product.
It is also known a process wherein a titanium alkoxide is used as a catalyst having high selectivity for the reaction (U.S. Pat. No. 3,686,268). But since the titanium alkoxide loses its activity even if a very small amount of water is present in the reaction system, it is necessary to remove the water from the reaction system previously. Further, there is a defect in that in order to separate the titanium alkoxide from the reaction mixture after the reaction, it is necessary to conduct a treatment with, for example, a sodium hydroxide aqueous solution so as to produce an inert product by hydrolysis. Further, the titanium alkoxide sometimes does not substantially show a catalytic activity for dihydric or higher polyhydric alcohols. Thus, it is not a general synthetic catalyst for methacrylic acid esters and is limited undesirably for its use.
Apart from the above-mentioned processes, it is proposed to use lithium hydroxide as a catalyst (Japanese Patent Unexamined Publication No. 54-61117). Since lithium hydroxide is relatively excellent in the activity and selectivity and has a good catalytic activity for polyhydric alcohols, it is considered to be a suitable catalyst. But according to the experiments conducted by the present inventors using lithium hydroxide monohydrate, it was found that it was possible to remove the catalyst by filtration after the reaction, but there was a defect in that it took a remarkably long filtering time due to poor filtering properties. Further, when the transesterification reaction was carried out by using a secondary or tertially alcohol as a starting material, there was a defect in that the reaction hardly proceeded due to a remarkably low catalytic activity.
On the other hand, Japanese Patent Unexamined Publication No. 54-61117 discloses that lithium hydroxide can be used in the form of either containing water or not containing water. Industrially available one is lithium hydroxide monohydrate, and this reference does not describe concretely how to dehydrate the lithium hydroxide monohydrate. According to known methods [Kagaku Binran (Handbook of Chemistry), Application edition, the Chemical Society of Japan, 1980, published by Maruzen, Ltd.], anhydrous lithium hydroxide can be obtained by (1) gradually heating lithium hydroxide monohydrate on a silver boat at 140.degree. C. in a hydrogen stream, or (2) drying lithium hydroxide monohydrate on phosphorus pentaoxide in vacuum for several days. Since these methods are complicated, require dangerous operations and can give only a very small amount of the anhydride at one time, it is very difficult to industrially use the anhydrous lithium hydroxide obtained by such methods.